It has been suggested that increases in the portion size of foods have contributed to excess energy intakes and thus the growing incidence of obesity. Few studies, however, have examined the relationship between portion size and food intake. It is probable that characteristics of the consumer, as well as of the type of food eaten, may influence how portion size affects energy balance. The energy density of foods (kcal per gram) is also likely to have a significant impact on the response to portion size. If energy density and portion size interact to affect intake, this will support the hypothesis that large portions of foods high in energy density have a major role in promoting excess energy intake. On the other hand, if increasing the portions of foods low in energy density also affects the amount eaten, this would suggest a strategy for reducing energy intake. To address these issues, four highly-controlled laboratory experiments are proposed. Experiments 1a, 1b, and 2 will test the effect on energy intake of incrementally increasing the portions of three different foods. Experiment 3 will investigate the interaction between portion size and energy density by using formulated foods which systematically vary in these two factors. Experiment 4 will test the impact of increasing the portions of all foods offered, both those high and low in energy density, over a two-day period. These studies will address the following specific aims: 1) to determine the effect on energy intake of increasing the portion size of different types of foods; 2) to identify characteristics of individuals which modulate the effect of portion size on intake; 3) to determine how portion size and energy density interact to influence intake; and 4) to determine the effect on energy intake of increasing the portion size of all foods served over two days. The proposed studies will provide fundamental data about the effect of portion size on intake of different types of foods, and about individuals likely to be most responsive to this effect. These data are critical for the development of effective strategies for weight management that incorporate the influence of portion size on energy intake.